THRESHOLD SPECTROSCOPIES AND THEIR PROSPECTS FOR MICROANALYSIS

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THRESHOLD SPECTROSCOPIES AND THEIR PROSPECTS FOR MICROANALYSIS

J. Kirschner

To cite this version:

J. Kirschner. THRESHOLD SPECTROSCOPIES AND THEIR PROSPECTS FOR MICROANAL- YSIS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-341-C2-344. �10.1051/jphyscol:1984277�.

�jpa-00223991�

THRESHOLD SPECTROSCOPIES AND THEIR PROSPECTS FOR MICROANALYSIS

J . Kirschner

IGV/KFA JUZich, P.O. Box 1913, 0 - 5 2 7 0 JZtZich, F.R.G.

Rgsurne - Le principe e t l e s propriPtGs des spectroscopies de seuil en analyse m f a c e sont present&. 11s sont compares d l a spectrom@trie Auger e t l e u r

r@solution l a t g r a l e prgvisible e s t discutee.

Abstract - The basic principle and the properties of threshold spectroscopies i n surface analysis are presented. Comparisons a r e made t o Auger microscopy and the prospects for l a t e r a l resolution are discussed.

I - INTRODUCTION

The 1 ow energy scanning electron microscope represents a major achievement in el ec- tron microscopy. Energies of about 100 eV and l e s s have been reached a t beam s i z e s of a few hundred nanometers /1,2/. A t low energies the SEN becomes surface sensitive and the knowledge of the elemental and chemical composition near the surface i s in- creasingly desirable. Apart from the importance of t h i s point in i t s own r i g h t , there appear t o e x i s t new contrast mechanisms i n the SEM, which are intimately re- l a t e d to the electronic and structural properties near the surface. /1,3,4/. The de- velopment of Scanning Auger Microscopy (SAM) was a major step towards surface micro- analysis, but t h i s technique requires a rather sophisticated high resolution el ec- tron energy analyzer - and the necessary space above the sample. There i s , however, a d i f f e r e n t c l a s s of spectroscopies, the so-called threshold spectroscopies /5/, t h a t are non-dispersive and consequently require 1 i t t l e more than a primary electron beam of variable energy. In the simplest case, t o be demonstrated below, i t i s suf- f i c i e n t to measure the specimen current only while slo:ly varying the beam energy.

While there has a 1 arge amount of work been done with broad" beams ( i .e. of more than 100 pm dia; for a review see ref. /5/) of which a few examples will be shown, there has no attempt y e t been made t o achieve l a t e r a l resolution. On the basis of experimental data i t will be shown t h a t the so-called Auger Electron Appearance Po- t e n t i a l Spectroscopy holds considerable promise f o r surface analysis in UHV scanning electron microscopes.

I1 - PRINCIPLE AND PROPERTIES OF THE THRESHOLD SPECTROSCOPIES

A schematic energy diagram of a metal1 i c sample together with a thermal source of electrons i s shown in Fig. 1. Electrons from the cathode are accelerated onto the sample by the voltage difference Up. When the primary energy Ep equals the binding energy EB of a core electron ( i n t h i s case of an L3 she1 1) a core hole may be gener- ated. The excited electron then resides a t or near the Fermi-level. T h i s i s also the case for the primary electron, as i t consumed a l l i t s energy in the excitation pro- cess. Consequently, the flux of el a s t i c a l l y ref1 ected electrons decreases a t an ex- c i t a t i o n threshold, similarly t o the classical Franck-Hertz experiment. This techni- que has been named Disappearance Potential Spectroscopy DAPS /6/. As a r e s u l t of the f i l l i n g of the core hole by electrons from occupied levels of lower binding energy, there may X-rays or Auger electrons be emitted. Experimentally, i t i s not necessary to analyze these p a r t i c l e s with respect t o energy, i t i s s u f f i c i e n t t o monitor t h e i r t o t a l flux which increases sl ightly a t an excitation thresh01 d. The re1 a t i v e l y small

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984277

C2-342 JOURNAL DE PHYSIQUE

s t r u c t u r e s ( o f t h e order o f o f the t o t a l f l u x ) a r e emphasized by e l e c t r o n i c d i f f e r e n t i a t i o n w i t h respect t o energy using the l o c k - i n technique. Depending on t h e type o f p a r t i c l e being monitored, t h e technique i s c a l l e d "SXAPS" ( S o f t x-ray Ap- pearance P o t e n t i a l Spectroscopy) o r "AEAPS" (Auger E l e c t r o n Appearance P o t a n t i a l Spectroscopy) /7/. I n t h e l a t t e r case, i t i s s u f f i c i e n t t o measure the specimen c u r - r e n t , being the d i f f e r e n c e between the primary beam c u r r e n t and t h e f l u x o f e l e c - t r o n s l e a v i n g t h e sample. The s i g n a l stems from Auger e l e c t r o n s and, even t o a l a r g - er degree, from secondary o r t e r t i a r y e l e c t r o n s e x c i t e d by them. Therefore essen- t i a l l y t h e same s i g n a l i s observed by using the output from a secondary e l e c t r o n de- t e c t o r i n s t e a d o f t h e specimen current. Though a l l these techniques a r e non-disper- sive, the energy r e s o l u t i o n i s q u i t e good. Experimentally, i t i s determined by the energetic width o f t h e primary beam (about 0.3 t o 0.5 eV). The t o t a l w i d t h o f t h e s p e c t r a l l i n e s i s a l s o determined by the d e t a i l e d shape o f the empty density o f s t a t e s and by t h e l i f e t i m e broadening o f the core level. The i d e n t i f i c a t i o n o f e l e - ments i s e a s i l y m d e by means o f the known b i n d i n g energies o f the core electrons. A p a r t i c u l a r advantage compared t o Auger spectroscopy i s t h a t t h e r e a r e only a few and narrow l i n e s per element t o be observed, so t h a t l i n e overlap i s rare. The surface s e n s i t i v i t y o f t h e t h r e s h o l d spectroscopies i s due t o the s h o r t i n e l a s t i c mean f r e e path f o r t h e primary electrons. A primary e l e c t r o n w i t h an energy c l o s e t o the t h r e s h o l d t h a t experienced a c h a r a c t e r i s t i c l o s s upon p e n e t r a t i n g the sample i s no more a b l e t o produce an e x c i t a t i o n . The depth o f i n f o r m a t i o n i s o f t h e order o f 1 nm. As a f u r t h e r b e n e f i t o f working a t threshold, b a c k s c a t t e r i n g e f f e c t s are ab-

sent, which m y cause considerable i n - t e n s i t y d i s t o r t i o n s i n SAM /8/. I n a d d i - t i o n t o element i d e n t i f i c a t i o n , t h e r e i s a l s o chemical i n f o r m a t i o n availab?,e due t o t h e e f f e c t o f "chemical s h i f t s . ^The

t h r e s h o l d energy changes s l i g h t l y when t h e e l e c t r o n i c surrounding o f a p a r t i c u - l a r atom i s changed by t h e formation o f a chemical bond. See f o r example Fig. 3 below. The l i n e shape a l s o contains i n - formation about the density o f empty states. When t h e energy conservation r u l e i s f u l f i l l e d , the t r a n s i t i o n r a t e wi 11 be determined by the empty d e n s i t y o f states accessible t o both electrons.

(See the schematic drawing on the r i g h t hand side o f Fig. 1.) When the l a t t e r i s the same f o r both e l e c t r o n s (exceptions f o r the r a r e earths are noteworthy /9/), the e x c i t a t i o n r a t e w i l l be p r o p e r t i o n a l t o t h e s e l f - c o n v o l u t i o n o f t h e empty density o f s t a t e s above t h e Fermi-level.

Fig. 1 - Schematic energy l e v e l diagram The empty density o f s t a t e s may be reco for t h r e s h o l d spectroscopies. vered by deconvolution /lo/.

I11 - ^EXAMPLES

An example f o r element i d e n t i f i c a t i o n i s given i n Fig. 2, showing the a n a l y s i s o f a s t a i n l e s s s t e e l surface using DAPS (upper panel) i n comparison t o an Auger a n a l y s i s o f t h e same surface (lower panel). The elements Fe, Cr, Mn, 0 and V are c l e a r l y i d e n t i f i e d i n DAPS, l i n e overlap being v i r t u a l l y absent. I n the Auger spectrum, t h e number o f l i n e s i s m c h l a r g e r and most o f them overlap. The elements V and Mn would a t best be d i s c e r n i b l e by computerized spectrum s u b s t r a c t i o n techniques. The s i g n a l s t r e n g t h depends t o a l a r g e degree on the empty density o f s t a t e s a t EF. Therefore C r i s s t r o n g i n t h e t h r e s h o l d spectroscopies, w h i l e 0 i s weak - r a t h e r the opposite i s observed i n AES. The v a r i a t i o n o f the s i g n a l i n t e n s i t y f o r d i f f e r e n t elements i s r a t h e r large. Strongest are the r a r e earths, f o l l o w e d by the t e c h n o l o g i c a l l y impor- t a n t 3d- and 4d-elements, w h i l e t h e noble metals y i e l d only f a i n t s i g n a l s /7/. A comparison o f AES, AEAPS and DAPS has been given i n r e f . Ill/.

I V - PROSPECTS FOR LATERAL RESOLUTION

I I I I

AES

W -

z

-1: _{I . . .... ..}

Mn ILMMl NI ILMMI

IKLLI

V ~ L ~ M I I I I

500 600 700 ENERGYleV

Microanalysis using e l e c t r o n beams i s determined by t h e compromise between c u r r e n t and beam diameter. Therefore t h e signal-to-noise r a t i o (S/N) i s the c r u c i a l para- meter. I n t h i s respect AEAPS i s the most promising t h r e s h o l d technique, as i t s SIN i s t h r e e t o f o u r orders o f magnitude b e t t e r than t h a t o f SXAPS, and s t i l l a f a c t o r o f 3 t o 10 b e t t e r than t h a t o f DAPS /11/. The p r i c e t o be p a i d f o r t h e s i m p l i c i t y o f t h e technique i s , t h a t i t s SIN i s i n many cases i n f e r i o r t o t h a t o f AES, i f t h e minimum d e t e c t a b i l i t y l i m i t a t a given beam diameter i s considered. The reason i s t h a t the i o n i z a t i o n cross s e c t i o n i s not maximum a t t h r e s h o l d b u t r a t h e r a t 3 t o 4 times the b i n d i n g energy, w i t h a slow decrease towards higher energy. For a given beam size, i n SAM the energy can be chosen higher and w i t h i t t h e beam current.

However, the power density i s frequently o f the same o r even greater importance i n microanalysis. When normalized t o t h e same power density, t h e S/N i n AEAPS was found t o be l a r g e r than i n AES by a f a c t o r o f 30 f o r La and by a f a c t o r o f 2 f o r Ti. For o x i d i z e d s t a i n l e s s s,teel, on t h e other hand, AES was

-

I I I I I

550 850 860 870 880 893

P r l m r y electron energy (eV)

Fig. 3 - Result o f Auger E l e c t r o n Appear- ance P o t e n t i a l Spectroscopy from c l e a n and o x i d i z e d N i . Note t h e "chemical s h i f t " o f - ^{1.1 eV.}

+ Fig. 2 - Comparison o f a n a l y s i s r e s u l t s from a s t a i n l e s s s t e e l surface as o b t a i n - ed by Disappearance P o t e n t i a l Spectrosco- py (DAPS; upper panel; i n f i r s t and se- cond d e r i v a t i v e mode) and by Auger spec- troscopy (lower panel ) .

A d e t a i l e d study o f S/N i n AEAPS has been given i n r e f . /11/, from which Fig. 4 i s reproduced. It shows t h e r e l a t i v e peak-to-peak i n t e n s i t i e s f o r T i and Fe and t h e s i g n a l - t o - n o i s e r a t i o s as a f u n c t i o n o f modulation amplitude. The SIN values are seen t o be o f the order o f 100 or b e t t e r a t the higher modulations l e v e l s , where the noise was only determined by t h e shot noise o f t h e e l e c t r o n beam. The s a t u r a t i o n i s due t o i n c r e a s i n g d i s t o r t i o n o f the l i n e shape w i t h i n c r e a s i n g modulation amplitude.

The inherent h i g h energy r e s o l u t i o n can t h e r e f o r e only be e x p l o i t e d a t a few V w h i l e f o r purely a n a l y t i c a l purposes higher l e v e l s can be t o l e r a t e d . From t h e @;nt An example f o r chemical e f f e c t s i s given i n Fig.

-

I n a d d i t i o n , t h e empty density o f states w i l l be modified by t h e presence o f a d d i t i - onal empty oxygen o r b i t a l s . Therefore the l i n e shape changes and an a d d i t i o n a l peak appears, marked by the arrow. Other examples m y be found i n ref. /5,6,7,12/.

C2-344 JOURNAL DE PHYSIQUE

[peak t o peak]

- 150 Fig. 4 - R e l a t i v e s t r e n g t h s ( l e f t hand s c a l e ) o f AEAPS s i g - n a l s f r o m c l e a n T i and Fe i n second de- r i v a t i v e mode and

-100 s i g n a l - t o - n o i s e r a - t i o s ( r i g h t hand s c a l e s ) as a f u n c t i o n o f m o d u l a t i o n a m p l i - tude. Primary beam c u r r e n t 10 pA, t i m e . 50 c o n s t a n t 0.3 sec.

o f view o f S/N t h e second d e r i v a t i v e mode i s unfavourable, due t o i t s low s e n s i t i v i - ty. I n t h e f i r s t d e r i v a t i v e mode much b e t t e r values a r e obtained. For Fe a t o n l y 0.8 Vpp modulation, S/N

-

As t h e shot n o i s e decreases w i t h t h e square r o o t o f t h e c u r r e n t , t h e p r i m a r y beam i n t n s i t y may be reduced by a f a c t o r o f 100, s t i l l l e a v i n g a S/N o f 10 a t l e a s t . A t 7

10- A beam c u r r e n t , even a t e n e r g i e s o f t h e o r d e r o f 1 keV o r l e s s , s u b s t a n t i a l l a t e r a l r e s o l u t i o n i s achievable. W i t h e l e c t r o s t a t i c lenses, beam diameters o f t h e o r d e r o f 10 pm a r e o b t a i n a b l e , w i t h magnetic lenses a r e s o l u t i o n o f perhaps 1 prn o r b e t t e r should be f e a s i b l e . T h i s seems t o be w i t h i n t h e r e a c h o f modern low v o l t a g e scanning e l e c t r o n microscopes. A few more t e c h n i c a l problems a r e d i s c u s s e d i n r e f .

, . A ,

I n summary, t h e main a t t r a c t i o n o f t h e t h r e s h o l d spectroscopies, and n o t a b l y o f AEAPS, l i e s i n t h e e x p e r i m e n t a l s i m p l i c i t y and t h e promise o f l a t e r a l l y r e s o l v e d s u r f a c e a n a l y s i s w i t h no a d d i t i o n a l equipment r e q u i r e d i n t h e a n a l y s i s chamber.

References

/1/ GELLER J., Scanning E l e c t r o n Microsc. 1983 ( i n p r e s s ) /2/ ICHINOKAWA T., Scanning E l e c t r o n Microsc. 1983 ( i n p r e s s ) /3/ LE GRESSUS C., Scanning E l e c t r o n Microsc. 1983 ( i n p r e s s )

/4/ NALL B.H., JETTE A.N., BARGERON C.B., Phys. Rev. L e t t . 3 (1982) 882

/5/ KIRSCHNER J., i n : E l e c t r o n Spectroscopy f o r S u r f a c e A n a l y s i s T o p i c s i n C u r r e n t P h y s i c s Vol 4, H. Ibach (ed.) ( S p r i n g e r Verlag, Heidelberg, New York, 1977) 59- 110

/6/ KIRSCHNER J., STAIB P., Appl. Physics 6 (1975) 99 /7/ PARK R.L., S u r f a c e Science 48 (1975) 8 n

/8/ KIRSCHNER J., Scanning E l e c t r o n Microsc. 1976/I, 215 /9/ KANSKI J., NILSSON P.O., Phys. Rev. L e t t . 43 (1979) 1185

/ l o / DOSE V., HAERTL A., J. Phys. F. 12 (1982) L147

/11/ KIRSCHNER J., LOSCH W., J. Vac. xi. ^Technol. 14 (1977) 1173 /12/ SCHEIDT H., GLOEBL M., DOSE V., S u r f a c e Science 123 (1982) L728 /13/ KIRSCHNER J., Scanning E l e c t r o n Microsc. 1983 ( i n p r e s s )